Circuit using choke coil and choke coil

ABSTRACT

A first winding of a choke coil is closely wound in a single layer on the outer periphery of a substantially cylindrical body portion of a first bobbin. A second winding is closely wound in a single layer over the first winding. A third winding is closely wound in a single layer on the outer periphery of a substantially cylindrical body portion of a second bobbin. A fourth winding is closely wound in a single layer over the third winding. The first, second, third and fourth windings are wound so as to mutually strengthen magnetic fluxes when an in-phase noise current flows. The first and second windings are connected to a pair of signal lines via which differential transmission communication is performed and on which a power supply current is sent out. The third and fourth windings are connected to a pair of signal lines via which differential transmission communication is performed and on which the power supply current returns. Thus, a circuit including a compact choke coil having large inductance and better high-frequency characteristics, and the choke coil is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit including a choke coil, andmore particularly, to a circuit having a choke coil inserted into asignal line having communication and power-provision functions, and alsorelates to a choke coil.

2. Description of the Related Art

In the related art, differential transmission circuits are used forcommunication. In differential transmission, a twisted pair line carriessignals having opposite phases, and the high/low level is determinedbased on which signal line has higher potential. For example, thecurrent most common LAN standard for personal computers is Ethernet(registered trademark), and a pulse transformer is provided as aninterface thereof. If high noise radiation is produced from a cable,common-mode choke coils are used before and after the pulse transformer.

One advantage of using a common-mode choke coil is that a restrictioneffect acts on common-mode noise without affecting the signals carriedwith opposite phases on the twisted pair line. In differentialtransmission, therefore, currents having the same magnitude flow withopposite phases in the twisted pair line, and the magnetic fluxesgenerated by the differential signal current are cancelled out in amagnetic core. On the other hand, the magnetic fluxes generated by anoise current flowing in-phase are mutually strengthened in a magneticcore.

In differential transmission communication, signals having a frequencyof 100 MHz or higher may be used, and the signal frequency and the noisefrequency band often overlap each other. A low-pass filter, such as anormal-mode choke coil, controls noise and signals at the same time, andis therefore difficult to use.

One known common-mode choke coil of the related art for preventing noisefrom entering a telephone line is described in Patent Document 1(Japanese Unexamined Utility Model Registration Application PublicationNo. 4-4712). As shown in FIG. 9, a common-mode choke coil 1 includes amagnetic core having two U-shaped core members 10 and 11, two bobbins 2and 3, and four windings 4, 5, 6, and 7.

The bobbins 2 and 3 have cylindrical body portions 2 a and 3 a arrangedin parallel to each other. Leg portions 10 b and 11 b of the coremembers 10 and 11 are inserted through holes 2 b and 3 b in thecylindrical body portions 2 a and 3 a, respectively. The core members 10and 11 form one closed magnetic path in which the leading ends of theleg portions 10 b and 11 b abut against each other in the holes 2 b and3 b.

The windings 4 and 5 are bifilar-wound in one layer on the cylindricalbody portion 2 a of the bobbin 2. The windings 6 and 7 are alsobifilar-wound in one layer on the cylindrical body portion 3 a of thebobbin 3. The windings 4 to 7 are wound so as to mutually strengthenmagnetic fluxes in the magnetic core when an in-phase current flows.

In the common-mode choke coil 1 having this structure, the number ofwinding portions in which the windings 4 and 5 or the windings 6 and 7are adjacent is only two in the horizontal direction shown in FIG. 9,and the stray capacitances caused at the adjacent wound portions areconnected in series a number of times corresponding to the number ofturns. Thus, the stray capacitance can be reduced, and the ability toprevent noise from entering the high band can increase.

However, the common-mode choke coil 1 described in Patent Document 1 hasa so-called bifilar-wound structure in which the windings 4 and 5 or thewindings 6 and 7 are alternately wound in one layer on the cylindricalbody portion 2 a or 3 a of the bobbin 2 or 3. Thus, there is a problemin that the number of turns of the windings 4 to 7 per unit length issmall, resulting in small inductance obtained compared to the size ofthe bobbins 2 and 3. A high-precision winding machine is required toproduce such a bifilar-wound structure. However, product failure stilloccurs due to disordered winding. Disordered winding greatly affects thehigh-frequency characteristics of the product.

Recently, a standard called IEEE 802.3af has been proposed by theInstitute of Electrical and Electronic Engineers. This standard definesa circuit having a power-provision circuit in a traditional differentialtransmission circuit, and also defines power provision via a signalline, such as a LAN cable for transmitting and receiving signals. Thisstandard is applied to devices, such as IP phones connected to LANcables and wireless LAN access points. When a common-mode choke coil isused for noise prevention on a signal line to be defined by thisstandard, the magnetic fluxes generated by a power supply current aregenerated in the direction in which they are strengthened in a magneticcore of the common-mode choke coil. Due to the magnetic fluxes generatedby the power supply current, the magnetic flux density of the magneticcore becomes close to a saturated magnetic flux density, and thecommon-mode choke coil inductance is reduced. The noise preventioneffect is therefore reduced. One approach for preventing an increase inthe magnetic flux density is to increase the cross-sectional area of themagnetic core. However, as the size of the magnetic core increases, theproduct size also increases. Moreover, the cost of the magnetic coreoccupies the majority of the product material cost. Thus, an increase inthe size of the magnetic core greatly affects the product price. If thenumber of turns of windings is small, small magnetic fluxes aregenerated in the magnetic core, and the core is less saturated. However,the inductance becomes small, and the noise prevention effect istherefore reduced.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a circuit including a compact chokecoil having large inductance, and a choke coil. More specifically,preferred embodiments of the present invention provide a compact chokecoil having large inductance and better high-frequency characteristicsthat can be inserted in a signal line circuit complying with IEEE802.3af.

A circuit including a choke coil according to a preferred embodiment ofthe present invention includes:

-   -   (a) first and second signal lines via which differential        transmission communication is performed and on which a power        supply current is sent out;    -   (b) third and fourth signal lines via which differential        transmission communication is performed and on which the power        supply current returns; and    -   (c) a choke coil having first, second, third, and fourth        windings, and a magnetic core constituting a closed magnetic        path in which the first, second, third, and fourth windings are        wound; wherein    -   (d) the first, second, third, and fourth windings are        electrically connected to the first, second, third, and fourth        signal lines, respectively; and    -   (e) the first winding and the second winding are wound in the        same direction so that magnetic fluxes generated in the magnetic        core are mutually strengthened when an in-phase noise current        flows, the third winding and the fourth winding are wound in the        same direction so that magnetic fluxes generated in the magnetic        core are mutually strengthened when an in-phase noise current        flows, and the first and second windings and the third and        fourth windings are wound so that magnetic fluxes generated in        the magnetic core are mutually strengthened when an in-phase        noise current flows.

With this unique structure, a signal line circuit having communicationand power-provision functions, more specifically, a circuit including achoke coil that is suitable for a signal line circuit complying withIEEE 802.3af, can be achieved.

A choke coil according to a preferred embodiment of the presentinvention is a choke coil that is inserted in a signal line havingcommunication and power-provision functions, including:

-   -   (f) first and second bobbins each having a substantially        cylindrical body portion;    -   (g) a first winding that is closely wound in a single layer on        the substantially cylindrical body portion of the first bobbin        and a second winding that is closely wound in a single layer        over the first winding;    -   (h) a third winding that is closely wound in a single layer on        the substantially cylindrical body portion of the second bobbin        and a fourth winding that is closely wound in a single layer        over the third winding; and    -   (i) a magnetic core having leg portions that are inserted        through holes in the substantially cylindrical body portions of        the first and second bobbins to constitute a closed magnetic        path; wherein    -   (j) the first winding and the second winding are wound in the        same direction so that magnetic fluxes generated in the magnetic        core are mutually strengthened when an in-phase noise current        flows, the third winding and the fourth winding are wound in the        same direction so that magnetic fluxes generated in the magnetic        core are mutually strengthened when an in-phase noise current        flows, and the first and second windings and the third and        fourth windings are wound so that magnetic fluxes generated in        the magnetic core are mutually strengthened when an in-phase        noise current flows. An insulating resin member, a        magnetic-powder-containing insulating resin member, a ferrite        member having a surface that is coated with insulating resin, a        metal member having a surface that is coated with insulating        resin, or a metal member may be disposed between the first        bobbin and the second bobbin.

With this unique structure, the first to fourth windings are closelywound in a single layer, and the number of turns per unit lengthincreases. Thus, large inductance can be obtained even if thesubstantially cylindrical body portions of the bobbins are short. Thenumber of wound portions in which the first and second windings or thethird and fourth windings are adjacent is only one in the verticaldirection shown in FIG. 2. Although the stray capacitances caused at theadjacent wound portion are connected in parallel only at the woundportion, the stray capacitances are small.

In the choke coil according to a preferred embodiment of the presentinvention, each of the first bobbin and the second bobbin includesflange portions at both ends of the substantially cylindrical bodyportion, and the outer peripheries of the flange portions of the firstbobbin are brought into contact with or engaged with the outerperipheries of the flange portions of the second bobbin. Thus, themechanical stress applied to one of the bobbins is distributed to theother bobbin, and the rigidity of the overall product increases. Achange in inductance due to the mechanical stress is also minimized.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external perspective view of a choke coil according to apreferred embodiment of the present invention.

FIG. 2 is a horizontal cross-sectional view of the choke coil shown inFIG. 1.

FIG. 3 is an electrically equivalent circuit diagram of the choke coilshown in FIG. 1.

FIG. 4 is a circuit diagram of a circuit in which the choke coil shownin FIG. 1 is connected to a signal line complying with IEEE 802.3af.

FIG. 5 is a schematic diagram for describing the effects and advantagesof the choke coil shown in FIG. 4.

FIGS. 6(A) to 6(D) are partially enlarged cross-sectional views showingengagement of the outer peripheries of flange portions of bobbins.

FIG. 7 is a horizontal cross-sectional view of a choke coil according toanother preferred embodiment of the present invention.

FIG. 8 is a perspective view of a metal member placed between thebobbins.

FIG. 9 is a horizontal cross-sectional view of a choke coil of therelated art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A circuit using a choke coil and the choke coil according to variouspreferred embodiments of the present invention will be described withreference to the accompanying drawings.

FIG. 1 is an external view of a common-mode choke coil, FIG. 2 is ahorizontal cross-sectional view of the choke coil, and FIG. 3 is anelectrical equivalent circuit diagram of the choke coil. A common-modechoke coil 31 preferably includes a magnetic core 50 having twosubstantially U-shaped core members 50 a and 50 b, two bobbins 32 and42, four windings 36, 37, 46, and 47, and a fitting plate 60.

The bobbins 32 and 42 include substantially cylindrical body portions 33and 43, and flange portions 34 and 35, and 44 and 45 at both ends of thesubstantially cylindrical body portions 33 and 43, respectively. Theflange portions 34, 35, 44, and 45 have pairs of lead terminals 53 a and54 a, 53 b and 54 b, 55 a and 56 a, and 55 b and 56 b, i.e., eightterminals. The bobbins 32 and 42 are arranged so that the substantiallycylindrical body portions 33 and 43 are substantially parallel to eachother. The bobbins 32 and 42 are preferably made of resin or othersuitable material.

The winding 36 is closely wound in a single layer on the outer peripheryof the substantially cylindrical body portion 33 of the bobbin 32. Thewinding 37 is closely wound in a single layer over the winding 36. Thewindings 36 and 37 are wound by the same number of turns in the samedirection so as to mutually strengthen magnetic fluxes when an in-phasenoise current flows. The winding 46 is also closely wound in a singlelayer on the outer periphery of the substantially cylindrical bodyportion 43 of the bobbin 42. The winding 47 is closely wound in a singlelayer over the winding 46. The windings 46 and 47 are wound by the samenumber of turns in the same direction so as to mutually strengthenmagnetic fluxes when an in-phase noise current flows. The windings 36and 37 and the windings 46 and 47 are wound by the same number of turnsso as to mutually strengthen magnetic fluxes when an in-phase noisecurrent flows.

Both ends of the winding 36 are electrically connected with the leadterminals 53 a and 53 b of the bobbin 32, and both ends of the winding37 are electrically connected with the lead terminals 54 a and 54 b.Both ends of the winding 46 are electrically connected with the leadterminals 55 a and 55 b of the bobbin 42, and both ends of the winding47 are electrically connected with the lead terminals 56 a and 56 b.

The core members 50 a and 50 b of the magnetic core 50 include armportions 51 a and 51 b, and leg portions 52 a and 52 b extendingsubstantially perpendicularly from both ends of the arm portions 51 aand 51 b, respectively. The leg portions 52 a and 52 b of the coremembers 50 a and 50 b are inserted in holes 33 a and 43 a in thesubstantially cylindrical body portions 33 and 43 of the bobbins 32 and42. The core members 50 a and 50 b define one closed magnetic path inwhich the leading ends of the leg portions 52 a and 52 b abut againsteach other in the holes 33 a and 43 a.

The core members 50 a and 50 b are preferably made of Mn—Zn or Ni—Znferrite, or both. Mn—Zn ferrite has high magnetic permeability, and cantherefore have larger inductance (several ten mH to several hundred mH)than Ni—Zn ferrite. An inductance of several ten mH to several hundredmH is required for suppressing a noise voltage from the low-frequencyband (several kHz). Ni—Zn ferrite has a better frequency characteristicof the magnetic permeability, and can therefore exhibit a largerinductance characteristic at a higher frequency (several ten MHz toseveral hundred MHz) than Mn—Zn ferrite. Both Mn—Zn ferrite and Ni—Znferrite may be used to have large inductance at a wide frequency band.

The fitting plate 60 having a substantially rectangular U-shapedconfiguration is engaged for robustly bringing the abutting surfaces ofthe core members 50 a and 50 b into close contact. The core members 50 aand 50 b may robustly be brought into close contact using adhesiveinstead of the fitting plate 60. The elements 32, 42, 50 a, 50 b, and 60are fixed by a fixing tool (not shown), or fixed by applying a minimumamount of adhesive or varnish (not shown) between the bobbins 32 and 42and the core members 50 a and 50 b.

The common-mode choke coil 31 having this structure has a large numberof turns per unit length because each of the windings 36, 37, 46, and 47is closely wound in a single layer. Thus, large inductance can beobtained even if the substantially cylindrical body portions 33 and 43of the bobbins 32 and 42 are short. The number of wound portions inwhich the windings 36 and 37 or the windings 46 and 47 are adjacent isonly one in the vertical direction shown in FIG. 2. Thus, the straycapacitance caused at the adjacent wound portion is small. Therefore, afour-terminal common-mode choke coil having better noise elimination atthe high-frequency band can be realized.

In IEEE 802.3af, it is necessary to eliminate noise from the lowfrequency region to the high frequency region, and the component thatforms the communication signal waveform overlaps the frequency band thatrequires noise prevention. Thus, large inductance, low leakageinductance, and high-frequency characteristics are demanded for thecommon-mode choke coil 31. If noise terminal voltage restrictions forthe low-frequency region (30 MHz or lower) are applied to acommunication line, the common-mode choke coil 31 is suitable for noiseelimination from the low frequency region to the high frequency region,and has effects of removing both a noise terminal voltage in the lowfrequency region (30 MHz or lower) and radiation noise in the highfrequency region (30 MHz or higher). The common-mode choke coil 31 istherefore suitable for the IEEE 802.3af standard.

A common-mode choke having a structure in which the wound area isdivided by a divider plate disposed on a substantially cylindrical bodyportion of a bobbin and windings are wound in different wound areas,which is referred to as a division-type common-mode choke coil, providesa large leakage magnetic flux. Therefore, this common-mode choke is notsuitable for the IEEE 802.3af standard, which requires small leakageinductance.

FIG. 4 shows a circuit in which the common-mode choke coil 31 isconnected to signal lines 71 to 74 complying with IEEE 802.3af for thepurpose of performing both communication and power-provision functions.The signal lines 71 to 74 are implemented by, for example, LAN cablesfor transmitting and receiving signals, which carry a power supplycurrent. In FIG. 4, reference numerals 61A and 61B denoteLAN-switch-side pulse transformers, reference numeral 62 denotes apower-provision source, reference numerals 65 and 66 denote connectors(for example, RJ-45 connectors), reference numeral 68 denotes a load,and reference numerals 69A and 69B denote data-terminal-side pulsetransformers.

The effects and advantages of the common-mode choke coil 31 will now bedescribed with reference to a schematic diagram shown in FIG. 5. Indifferential transmission communication, same-magnitude differentialsignal currents having opposite phases flow in two pairs of windings 36and 37, and 46 and 47. A magnetic flux φ1 that is generated in themagnetic core 50 by flowing a signal current in the winding 36 of thepair of windings 36 and 37, and a magnetic flux φ1 that is generated inthe magnetic core 50 by flowing a signal current in the other winding 37are generated with the same magnitude in opposite directions. Thus, themagnetic fluxes φ1 and φ1 are cancelled out. The same applies to thepair of windings 46 and 47.

The phenomenon that magnetic fluxes are cancelled out occursindependently in the pair of windings 36 and 37 and the pair of windings46 and 47. Therefore, if two different differential signal currents arecarried by the two pairs of windings 36 and 37, and 46 and 47 at thesame time, the interference due to magnetic coupling does not occur inthe magnetic core 50.

A combination (parallel connection) of the windings 36 and 37 is used asa line on which the power supply current is sent out, and a combination(parallel connection) of the windings 46 and 47 is used as a line onwhich the power supply current returns. In this case, a sum of the powersupply currents applied to the windings 36 and 37 and a sum of the powersupply currents applied to the windings 46 and 47 are the same inmagnitude and opposite in phase. Thus, a magnetic flux φ2 that isgenerated in the magnetic core 50 via the windings 36 and 37 and amagnetic flux φ2 that is generated in the magnetic core 50 via thewindings 46 and 47 are cancelled out. Therefore, the magnetic core 50 isnot magnetically saturated. In the magnetic core 50 that is small, theinductance can increase as the number of turns of the windings 36, 37,46, and 47 increases.

Accordingly, the functionality of the common-mode choke coil can besufficiently achieved. The combination of the windings 36 and 37 and thecombination of the windings 46 and 47 allow a large tolerant current toflow in the lines.

In the common-mode choke coil 31, when a common-mode (in-phase) noisecurrent Ic flows in the windings 36, 37, 46, and 47, magnetic fluxes φcare generated in the same direction in the magnetic core 50 via thewindings 36, 37, 46, and 47. The magnetic fluxes φc turn in the magneticcore 50 while they are mutually strengthened. Therefore, the impedancebecomes large with respect to the common-mode noise current Ic, and thecommon-mode noise current Ic is minimized. It is presumed that thecommon-mode noise current Ic is about several mA at the peak and thepower supply current is about several hundred mA.

As indicated by circle portions S shown in FIG. 2, in this preferredembodiment, the outer peripheries of the flange portions 34 and 35 ofthe bobbins 32 are brought into contact with the outer peripheries ofthe flange portions 44 and 45 of the bobbin 42. Thus, the mechanicalstress applied to one of the bobbins is distributed to the other bobbin,and the rigidity of the overall common-mode choke coil 31 increases. Themechanical stress is not locally applied to the magnetic core 50, andthere is no fear that the abutting surfaces of the core members 50 a and50 b will be out of position or a gap will occur. Therefore, theeffective magnetic permeability of the magnetic core 50 is not prone tochange, and a stable inductance characteristic can be obtained. Bychanging the sizes of the flange portions 34, 35, 44, and 45, thedistance between the windings 36 and 37 and the windings 46 and 47 canbe adjusted, and the electromagnetic interference and the insulatingcharacteristic can be adjusted.

In this case, not only are the outer peripheries of the flange portions34 and 35 and the outer peripheries of the flange portions 44 and 45contacted but the flange portions 34 and 35 and the flange portions 44and 45 are also engaged with each other, as shown in, for example, FIGS.6(A) to 6(D), which is more effective.

Generally, common-mode choke coils have a slight normal-mode leakageinductance component, and have a further advantage of removingnormal-mode noise. However, if common-mode noise and strong normal-modenoise are caused to flow in a signal (power supply) line, common-modechoke coil parts and normal-mode choke coil parts must be used to takenoise measurements. In a common-mode choke coil having a relativelylarge normal-mode leakage inductance component, the leakage magneticflux can affect a peripheral circuit. In this case, a magnetic shield isrequired to be placed over the outer circumference of the common-modechoke coil.

Accordingly, as shown in FIG. 7, a magnetic-powder-containing insulatingresin member 80 having a relative magnetic permeability of about 1 orhigher (e.g., about 2 to about several tens) is placed between the twoadjacent bobbins 32 and 42 of the common-mode choke coil 31. Themagnetic-powder-containing insulating resin member 80 is brought intocontact with or is engaged with the outer peripheries of the flangeportions 34, 35, 44, and 45 of the bobbins 32 and 42. Themagnetic-powder-containing insulating resin member 80 is preferably madeby kneading Ni—Zn ferrite of, for example, about 80 wt % to about 90 wt% and nylon or polyphenylene sulfide resin.

The magnetic-powder-containing insulating resin member 80 is easilyprocessed and has an insulating property. Thus, no insulating spacer isrequired between the core members 50 a and 50 b.

The magnetic-powder-containing insulating resin member 80 increases theeffective magnetic permeability of a normal-mode magnetic path, andmagnetic fluxes φ are concentrated in the magnetic path having higheffective magnetic permeability (the magnetic-powder-containinginsulating resin member 80 and the core members 50 a and 50 b). Thus,the common-mode choke coil 31 having a large normal-mode inductancecomponent and capable of also eliminating strong normal-mode noise canbe achieved, and any adverse effect of the leakage magnetic flux on aperipheral circuit can be minimized.

The value of the normal-mode inductance component depends upon thecontact area of the core members 50 a and 50 b and themagnetic-powder-containing insulating resin member 80, the gaptherebetween, the relative magnetic permeability of themagnetic-powder-containing insulating resin member 80, etc. In thecommon-mode choke coil 31, as the normal-mode inductance componentincreases, the core members 50 a and 50 b are readily saturated. Theextent to which the normal-mode inductance component can increasedepends upon the characteristics (the saturation characteristic, therelative magnetic permeability, etc.) of the used core members 50 a and50 b and the current flowing in the common-mode choke coil 31. That is,it is necessary to increase the normal-mode inductance component withina prescribed operating range of the common-mode choke coil 31 using themagnetic-powder-containing insulating resin member 80 so that the coremembers 50 a and 50 b are not saturated.

The magnetic-powder-containing insulating resin member 80 between thetwo bobbins 32 and 42 can extend the distance of insulation between thewindings 37 and 47, and can effectively utilize the space of thecommon-mode choke coil 31 to reduce the size.

In place of the magnetic-powder-containing insulating resin member 80, aferrite member having a surface that is coated with insulating resin maybe used. This ferrite member (preferably made of Mn—Zn or Ni—Zn ferrite)also achieves similar effects and advantages to those of themagnetic-powder-containing insulating resin member 80.

Alternatively, an insulating resin member may be used instead of themagnetic-powder-containing insulating resin member 80. The distancebetween the windings 36 and 37 and the windings 46 and 47 can beadjusted depending upon the thickness of the insulating resin member,and the electromagnetic interference and the insulating characteristiccan efficiently be improved.

In place of the magnetic-powder-containing insulating resin member 80, ametal member 90 shown in FIG. 8 may be used. The metal member 90 hasground lead terminals 91, and the ground lead terminals 91 are solderedto a ground pattern of a printed circuit board. Thus, the metal member90 functions as an electromagnetic shield for suppressing theelectromagnetic interference between the windings 36 and 37 and thewindings 46 and 47. A surface of the metal member 90 may be coated withinsulating resin to increase the insulating characteristic.

The present invention is not limited the illustrated preferredembodiments, and a variety of modifications may be made withoutdeparting from the scope of the invention. For example, a square-shapedintegrated core or a double-square-shaped integrated core may be used asa magnetic core, and a bobbin having a gear divided into two or morepieces may be used as a bobbin.

According to preferred embodiments of the present invention, therefore,a circuit including a compact choke coil having large inductance can berealized. The choke coil of preferred embodiments of the presentinvention has a large number of turns per unit length because first tofourth windings are closely wound in a single layer. Thus, largeinductance can be obtained even if a bobbin has a short substantiallycylindrical body portion. Moreover, the stray capacitance caused at awound portion in which the first and second windings or the third andfourth windings are adjacent is small. Therefore, a compact choke coilhaving large inductance and better high-frequency characteristics thatcan be inserted in a signal line circuit complying with IEEE 802.3af canbe provided.

It should be understood that the foregoing description is onlyillustrative of the present invention. Various alternatives andmodifications can be devised by those skilled in the art withoutdeparting from the present invention. Accordingly, the present inventionis intended to embrace all such alternatives, modifications, andvariations that fall within the scope of the appended claims.

1-4. (canceled)
 5. A circuit including a choke coil, comprising: firstand second signal lines via which differential transmissioncommunication is performed and on which a power supply current is sentout; third and fourth signal lines via which differential transmissioncommunication is performed and on which the power supply currentreturns; and a choke coil having first, second, third, and fourthwindings, and a magnetic core constituting a closed magnetic path inwhich the first, second, third, and fourth windings are wound; whereinthe first, second, third, and fourth windings are electrically connectedto the first, second, third, and fourth signal lines, respectively; thefirst winding and the second winding are wound in the same direction sothat magnetic fluxes generated in the magnetic core are mutuallystrengthened when an in-phase noise current flows, and the third windingand the fourth winding are wound in the same direction so that magneticfluxes generated in the magnetic core are mutually strengthened when anin-phase noise current flows; and the first and second windings and thethird and fourth windings are wound so that magnetic fluxes generated inthe magnetic core are mutually strengthened when an in-phase noisecurrent flows.
 6. The circuit according to claim 5, wherein each of thefirst bobbin and the second bobbin includes flange portions at both endsof the substantially cylindrical body portion thereof, and outerperipheries of the flange portions of the first bobbin contact with orare engaged with outer peripheries of the flange portions of the secondbobbin.
 7. The circuit according to claim 5, wherein one of aninsulating resin member, a magnetic-powder-containing insulating resinmember, a ferrite member having a surface that is coated with insulatingresin, a metal member having a surface that is coated with insulatingresin, and a metal member is placed between the first bobbin and thesecond bobbin.
 8. The circuit according to claim 5, wherein the magneticcore includes two substantially U-shaped core members.
 9. The circuitaccording to claim 5, further comprising two bobbins includingsubstantially cylindrical body portions and flange portions at both endsof the substantially cylindrical body portions.
 10. The circuitaccording to claim 9, wherein the flange portions include pairs of leadterminals connected to a respective one of the first, second, third andfourth windings, and the bobbins are arranged so that the substantiallycylindrical body portions are substantially parallel to each other. 11.The circuit according to claim 5, wherein the first and second windingsare wound by the same number of turns in the same direction so as tomutually strengthen magnetic fluxes when an in-phase noise currentflows.
 12. The circuit according to claim 5, wherein the third andfourth windings are wound by the same number of turns in the samedirection so as to mutually strengthen magnetic fluxes when an in-phasenoise current flows.
 13. The circuit according to claim 5, wherein thefirst and second windings, and the third and fourth windings, are woundby the same number of turns so as to mutually strengthen magnetic fluxeswhen an in-phase noise current flows.
 14. The circuit according to claim5, further comprising two bobbins including substantially cylindricalbody portions having holes provided therein, wherein the magnetic coreincludes two substantially U-shaped core members, the core membersinclude arm portions and leg portions extending substantiallyperpendicularly from both ends of the arm portions, and the leg portionsare inserted in the holes in the substantially cylindrical body portionsof the bobbins.
 15. The circuit according to claim 14, wherein the coremembers define one closed magnetic path in which leading ends of the legportions abut against each other in the holes.
 16. The circuit accordingto claim 15, further comprising a fitting plate having a substantiallyrectangular U-shaped configuration and arranged to bring abuttingsurfaces of the core members into close contact with each other.
 17. Thecircuit according to claim 5, wherein the choke coil operates accordingto the IEEE 802.3af standard.
 18. A choke coil that is inserted in asignal line having communication and power-provision functions,comprising: first and second bobbins each having a substantiallycylindrical body portion; a first winding that is closely wound in asingle layer on the substantially cylindrical body portion of the firstbobbin and a second winding that is closely wound in a single layer overthe first winding; a third winding that is closely wound in a singlelayer on the substantially cylindrical body portion of the second bobbinand a fourth winding that is closely wound in a single layer over thethird winding; and a magnetic core having leg portions that are insertedthrough holes in the substantially cylindrical body portions of thefirst and second bobbins to define a closed magnetic path; wherein thefirst winding and the second winding are wound in the same direction sothat magnetic fluxes generated in the magnetic core are mutuallystrengthened when an in-phase noise current flows; the third winding andthe fourth winding are wound in the same direction so that magneticfluxes generated in the magnetic core are mutually strengthened when anin-phase noise current flows; and the first and second windings and thethird and fourth windings are wound so that magnetic fluxes generated inthe magnetic core are mutually strengthened when an in-phase noisecurrent flows.
 19. The choke coil according to claim 18, wherein each ofthe first bobbin and the second bobbin includes flange portions at bothends of the substantially cylindrical body portion thereof, and outerperipheries of the flange portions of the first bobbin contact with orare engaged with outer peripheries of the flange portions of the secondbobbin.
 20. The choke coil according to claim 18, wherein one of aninsulating resin member, a magnetic-powder-containing insulating resinmember, a ferrite member having a surface that is coated with insulatingresin, a metal member having a surface that is coated with insulatingresin, and a metal member is placed between the first bobbin and thesecond bobbin.
 21. The choke coil according to claim 18, wherein themagnetic core includes two substantially U-shaped core members.
 22. Thechoke coil according to claim 18, wherein the first and second bobbinsinclude flange portions at both ends of the substantially cylindricalbody portions and the flange portions include pairs of lead terminalsconnected to a respective one of the first, second, third and fourthwindings, and the first and second bobbins are arranged so that thesubstantially cylindrical body portions are substantially parallel toeach other.
 23. The choke coil according to claim 18, wherein the firstand second windings are wound by the same number of turns in the samedirection so as to mutually strengthen magnetic fluxes when an in-phasenoise current flows.
 24. The choke coil according to claim 18, whereinthe third and fourth windings are wound by the same number of turns inthe same direction so as to mutually strengthen magnetic fluxes when anin-phase noise current flows.
 25. The choke coil according to claim 18,wherein the first and second windings, and the third and fourthwindings, are wound by the same number of turns so as to mutuallystrengthen magnetic fluxes when an in-phase noise current flows.
 26. Thechoke coil according to claim 18, wherein the substantially cylindricalbody portions of the first and second bobbins have holes providedtherein, the magnetic core includes two substantially U-shaped coremembers, the core members include arm portions and the leg portionsextend substantially perpendicularly from both ends of the arm portions,and the leg portions are inserted in the holes in the substantiallycylindrical body portions of the bobbins.
 27. The choke coil accordingto claim 26, further comprising a fitting plate having a substantiallyrectangular U-shaped configuration and arranged to bring abuttingsurfaces of the core members into close contact with each other.
 28. Thechoke coil according to claim 18, wherein the choke coil operatesaccording to the IEEE 802.3af standard.